Variable voltage motor control system



May 20, 1947. s. E. KING ETAL VARIABLE VOLTAGE MOTOR CONTROL SYSTEMFiled Oct. 17, 1944 2 Sheets-Sheet 1 INVENTORS 6eoryehhyandMflfan/ifrmfla x ATTORNEX May 20, 1947. G. E. KING ETAL VARIABLE VOLTAGEMOTOR CONTROL SYSTEM Filed Oct. 17, 1944 2 Sheets-Sheet 2 3 Vol/a OZfimpwne Field Cur/en) I Turns 7 L2 Ll I I ea:

WITNESSES: INVENTORS Geo/"ye E A709 and I My/l'gYfn l'i/Zrmbo/s,

ATTORNEY Patented May 20, 1947- VARIABLE VOLTAGE MOTOR CONTROL SYSTEMGeorge E. King,

Swissvale, and William H.

Formhals, Forest Hills, Pa., asslgnors to Westinghouse ElectricCorporation, East Pittsburgh,

Pa., a corporation oi Pennsylvania Application October 17, 1944, SerialNo. 559,068

8 Claims.

The present invention relates to variable voltage control systems and,mole particularly, to electric motor control systems for operating orautomatically controlling the operation of motors connected to varioustypes of mechanical loads. In certain of its aspects this invention isrelated to the copending applications of George E. King, Serial No.559,065, filed on the same date as this application and entitled Controlsystems, to Serial No. 559,066, filed on the same date as thisapplication and entitled Control systems, and to Serial No. 559,067,also filed on the same date as this application and entitled Controlsystems.

This invention provides certain improvements in variable voltage controlsystems in which rotating regulators are employed to regulate certainelectrical quantities of the system, and the invention as hereinafterdescribed and-as illustrated in the. drawings, is specifically directedto the regulation of the speed of rotation of a directcurrent motortogether with provisions for limitaccelerating and braking current.

ing the motor current whether the motor is operating normally as a motoror during regenerative periods when the motor is being overhauled by itsmechanical load and driven as a generator. It will be apparent to oneskilled in the art that the invention is not limited to speed regulationalone since various modifications of the system may be made to controlthe motor torque or power in conjunction with the current limitingfeatures mentioned. It will further be apparent that the invention isnot necessarily limited to motor control since automatic regulation ofthe voltage, current or electrical power supplied to an electrical loadof substantially any type susceptible of regulation may be had.

In the single embodiment of the invention illustrated in the drawingsthe variable voltage control system is particularly adapted forcontrolling the speed of a motor used to drive a centrifugal castingmachine. Such a machine usually has fixed running and creeping speeds.After the metal is poured into the mould to be rotated, the machine isaccelerated, by way of example, in 15 minutes to the running speed, atwhich speed it may be operated for about 50 minutes. ihe machine is thendecelerated and brought to rest in approximately a 10 minute interval oftime. It is then again started and brought up to the creeping speedwhere it is operated for about 100 minutes after which it is againbrought to rest and in due course of time the completed casting isremoved. v

The running speed of the centrifugal casting machine is preferablyconstant.

regulation at these different speeds is essential.

2 machine is preferably constant. Likewise the slow or creeping speed ofthe centrifugal casting Thus speed In the instant application such speedregulation is obtained by means of a rotating regulator. Currentlimiting control by means of a second rotating regulator is alsoprovided to limit the The rotating regulator med to control the motorspeed is so connected that it circulates current through the fieldwindings of the main generator supplying the motor! This current iscirculated in such a direction with respect to the normal supplycurrents circulating in these fields that the current supply to themotor by the main generator is of a value which tends to maintain thepreselected constant speed. The second rotating regulator mentionedwhich provides protection against excessive motor armature currents,modifies the control of the speed controlling rotating regula tor. Thiscurrent limiting regulator is operative to modify the action of thespeed controlling rotating regulator on the system only during .periodswhen the motor armature currents have exceeded a preselected maximumvalue. The combined actions of the two rotating regulators is such thatthe motor is always accelerated in the smallest interval of time that ispossible without the motor armature current exceeding the preselectedmaximum value.

The creeping speed is very slow and may, for example, be about 1.2% ofthe full field speed of the main drive motor. Speed regulation of themotor at this slow speed is obtained in a manner similar to that at therunning speed of the motor. Suitable control means, however, areprovided to recalibrate the system to insure good stability of therotating regulators under this new condition.

In order to simplify the drawings and the accompanying descriptivedisclosure the form of the invention illustrated is shown withoutreversing facilities. Reversing may be obtained by simply addingreversing contactors. Such expedients are well known in the art and anysuitable form of reversing equipment may be utilized without departingfrom the spirit and scope of the teachings of this invention.

A principal object of this invention is to provide a variable voltagedrive including a motor in which automatic speed regulation of the motoris provided in conjunction with automatic regulation of the motorcurrent.

Another object of this invention is to provide a variable voltage drivein which at least two selectively obtained constant running speeds for amotor are provided in conjunction with current limiting protection forthe motor and in which one of these speeds is variable over apreselected speed range.

Still another object of this invention is to provide a variable voltagedrive of the character referred to which provides a wide range of speedregulation for a motor in conjunction with protection against excessivemotor currents.

A specific object of this invention is to provide a variable voltagedrive utilizing rotating regulators for regulating the speed of a motorand limiting the motor current in which the rotating regulatorcontrolling the motor currents has its electrical output effectivelyblocked over a certain range of motor currents by an oppositely appliedpotential 01 a preselected value.

Other objects and advantages will become apparent upon a study of thefollowing disclosure when considered in conjunction with theaccompanying drawin s, in which:

Figure 1 schematically illustrates a variable 4 voltage drive providingprotection against excessive currents in which constant speeds of themotor controlled by the variable voltage system are selectivelyobtained;

Fig. 2 diagrammaticallyillustrates the connections of the two rotatingregulators and their associated elements in the system shown in Fig. 1;

Fig. 3 is a curve graphically illustrating the operating characteristicsof one of the rotating regulators utilized in this invention; and

Fig. 4 is a curve graphically illustrating the operating characteristicof the other rotating regulator utilized in this invention.

Referring now to Fig. l of the drawings, the invention illustratedtherein comprises generally a main drive motor M which is mechanicallyconnected to drive the mentioned centrifugal casting machine (notillustrated). The armature winding of this motor is connected in serieswith the armature winding of a main generator G. A speed regulatinggenerator RS is'utilized to control the electrical output of the maingenerator G. A current limiting regulating generator RCL is utilized tolimit the current circulating in the series motor generator armaturecircuit within preselected maximum values. An exciter E is provided tosupply a constant electrical quantity to various elements of the system.The exciter E, the current limiting regulator RCL, the speed regulatorRS and the main generator G are driven at a constant speed by anysuitable constant speed prime mover or system of prime movers. As shownby way of illustration but not limitation, these machines are connectedto a common shaft to be driven by a single prime mover. The constantspeed prime mover, however, is not shown in an effort to simplify theillustration of the invention.

The main motor M is provided with a control field winding MF and aseries connected commutating field winding MCF.

The main generator G is provided with a commutating field winding GCF,also series connected. A differential field winding GDF is provided forthe main generator and utilized at the end of braking periods of themotor to buckdown or neutralize the residual voltage of the maingenerator. This main generator is further provided with control fieldwindings GSFI and GSF2 which are connected, as more easily seen thesefield windings.

in Fig. 2, in opposite legs of a conventional Wheatstone bridge circuit.

The speed regulating generator RS is provided with a plurality of fieldwindings. Of these windings, those designated RSI and R82 are con nectedin series with the main generator field winding GSFi in one leg of thebridge, and those designated RS3 and RS4 are connected in series withthe main generator field winding GSF2 in the opposite leg of the bridgecircuit. The field winding RS5 for this machine is connected across themain generator commutating and differential field winding and is thusexcited by a current proportional to that flowing through The fieldwinding desig nated RS6 is connected in series with the arma turecircuit of the current limiting regulating generator REL. The fieldwinding designated R81 is connected in series with the winding RCLI ofthe current limiting generator and the resistor Rl 9 and this seriescircuit is connected across the motor armature terminals. These serieselements are thus excited by currents proportional to the motor armatureterminal voltage.

The current limiting regulating generator RCL -is provided with threecontrol field windings RCLI, RCL2 and RCL3. The winding designated RCLZis connected in parallel with the winding RS5 of the speed regulatinggenerator. The winding RCLI under certain operating conditions isconnected in series circuit with the speed controlling rheostat SR whichin turn, through a group of series connected resistors, is connected toone input terminal of the electrical bridge circuit. The winding RQL3 asabove noted is in series with the winding RS1. across the motor armatureterminals. The windings RCLI and RCLI are additively connected and thewinding RCL3 is differentially connected with respect to the windingsRCLI and RCL2.

The exciter E is provided with a single control field winding EF whichis connected in shunt circuit with the-exciter armature. This machinemay be provided with any suitable field winding arrangement since per seit forms no part of this invention.

The control equipment which cooperatively functions to provideselectively obtained operations of the system, as well as functioning inpart to protect the system, comprises a main contactor MC which isutilized to complete the series motor generator armature circuit inpreparation for starting.

A field loss relay FL has its operating coil connected in series withthe motor field winding MF and thus senses a loss of motor field.

A relay R has its coil connected across the motor commutating field MCFand is thus energized by the drop across this field winding.

Voltage relay IVR has its coil connected across the motor armatureterminals and is energized according to the voltage drop thereaeross.This relay picks up when the voltage across the motor armature terminalsis fairly low for example that corresponding to the creeping speed ofthe motor.

Voltage relay 2VR which, as shown, is connected in parallel with therelay IVR is adjusted to pick up at some voltage across the motorarmature terminals which corresponds to a low speed say 30% of topspeed. This relay recalibrates the current limiting regulating generatorto limit initial starting currents. This limits the starting torque ofthe motor to desirable values.

Voltage relay WR is energized by the voltage I drop across the generatorarmature terminals.

Differential field relay DF is energized in an amount proportional tothe exciter voltage. While this relay, as a number of other relays inthis system, are not shown connected to the exciter, this is done tosimplify the circuit arrangement illustrated. It will be understood thatany suitable energizing potential for these relays is satisfactory. Thisdiflerential field relay DF, when deenergized, connects the generatordifferential field GDF across the generator armature terminals thusneutralizing the generator residual voltage.

The relay DFR, energized by a suitable energizing potential as is the.relay DF just mentioned, is utilized to connect the armature of the,

tain of the field circuits oi! the speed regulating generator RS, toshunt the field RCL3 from its series connection with the field RS! andat the same time to select the proper potential to be applied across theinput terminals of the electrical bridge circuit during creepingoperation of the motor.

A run relay RR is provided which functions alternately with the creeprelay to provide the necessary control of the motor for the runningspeed thereof.

An accelerating and braking relay AB is provided to selectively obtainthe desired accelerating and braking connections.

sequentially operating accelerating relays IAR, 3AR and 4AR, whichfunction, respectively, in conjunction with the auxiliary acceleratingrelays IA, 3A and 4A, are provided tocontrol the accelerating currentssupplied to the electrical bridge. Similarly, braking relays IBR, 3BRand 4BR are provided to control the braking currents supplied to theelectrical bridge.

A low voltage relay LV, which is preferably energized according to theexciter potential, is utilized to deenergize the AB and CR relays in theevent that the exciter potential drops below a predetermined value.

A master switch MS is provided to control the CR and RR relays. Invthecreep position of this master switch the CR relay is energized and thusthe motor is operated at creeping speeds. In the run position of thismaster switch the RR relay is eventually energized through theaccelerating and braking relays and the motor is operated at its runningspeed.

The main contactorMC is provided with a. plurality of contacts of whichthe contact MCI is the main contact and is utilized to complete themotor generator series armature circuit. The back contact MC2 thereof isconnected in series with a resistor R1 across the input terminals I and2 of the electrical bridge circuit. This circuit when closed by thecontacts M02 in effect forms a discharge path for the field windingsconnected in the opposite legs of the electrical bridge. The contactmembers M03 complete a partial energizing ircuit for the relay DFR. Thecontact members MCl complete a partial energizing circuit for the lowvoltage relay LV. The contact members MCS shunt the start" push buttonwhich is connected in series with the stop push button and the coil ofthis main contactor. These contacts thus form a holding circuit for thecoil of this contactor.

The relay R is provided with a single contact RI which, in part, formsan energizing circuit for the timing relay TR.

The voltage relay IVR is provided with a single contact IVRI whichshunts the contact RI and also provides a partial energizing circuit forthe timing relay TR.

The voltage relay 2VR is provided with a single contact 2VRI whichshunts the resistor RH connected in series with the acceleratingrheostat AR.

The voltage relay SVR is provided with a single contact 3VRI which isconnected in shunt circuit with the previously mentioned IVRI and RIcontacts and, like these contacts, completes a partial energizingcircuit for the timing relay TR.

The differential field relay DF has its back contact DFI connected tocomplete the connection of the generator difierential field winding GDFwhen this differential field relay is deepergized, The contact membersDF2 partially complete a circuit for the coil of the run relay RR.Contact members DF3 partially complete energizing circuits for the creeprelay CR and for the accelerating and braking relay AB.

The relay DFR is provided with a single contact DFRI which connects thearmature of the speed regulating generator RS across the outputterminals of the electrical bridge circuit.

The timing relay TR is provided with a contact TRI which is connected toa point of positive potential and the conductor 5. When closed thiscontact supplies a positive potential to the conductor 5 which isutilized to energize the relays DF, DFR and 4A. It also has a backcontact TR2 which is connected in series with the reset push button.Thus when this contact is open operation of the reset push button doesnot energize the coil of the MC contactor. It is thus impossible toclose the motor generator armature circuit if a voltage exists acrossthe motor or generator armatures.

The run contactor RC is provided with a plurality of contacts numberedRCI to RC1. The contacts RCI and RC4 are utilized to shunt certain ofthe resistors and field windings from all four legs of the electricalbridge, and at the same time to shunt certain of the resistors connectedin series with the armature of the speed regulating generator RS fromthe bridge. In other words this relay changes the calibration of thebridge circuit.

The creep relay CR is provided with a plurality of contacts designatedCRI to CR6. The contact members CRI shunt the resistor RI! and thuschange the calibration of the parallel connected field windings RS5 andRCL2. The contact members CR2 shunt the field winding RCL3 and itsseries resistor RIB, thereby effecting energization of the field windingRS1 with the full voltage drop across the motor armature terminals andin efiect disconnecting the-winding RCL3 from the circuit. Contactmembers CR3, in conjunction with contact members CR4, connect the speedrheostat SR and its series connected resistors RI4, RI5 and RIB acrossthe exciter terminals. At the same time the contact members CR3 connecta portion of the resistor RIB in shunt circuit relationship with theinput terminals of the electrical bridge circuit. The bridge circuit isthus in effect energized by the voltage drop across the shunt connectedportion of the resistor RIG. The contact members CR5, which are backcontacts, complete a. partial energizing circuit for the coil of the runcontactor RC, The back contact member CR6 completes a partial energizingcircuit for the coil of the accelerating and braking relay AB.

The run relay RR is provided with the contacts RRI to RR3. The contactRRI connects the control field RCLI for the current limiting regulatinggenerator RCL in series with a selected portion of the speed rheostatSR. The contact members RRZ when closed supply positive potential to theaccelerating and braking relays. The contact members RR3, when closed,partially complete an energizing circuit for the accelerating relay IAR.

The accelerating and braking relay AB is provided with a plurality ofcontacts designated ABI to AB4. The contact members ABI, which are backcontacts, connect the resistor RI2 in series with the braking rheostatBR and its series connected resistors RI I, R9 and RIO. Contact membersAB2 function in part to complete the energizing circuit for theaccelerating rheostat AR and its series resistor RI3. The acceleratingand braking rheostats and their associated resistors and contacts areconnected in parallel relationship and this parallel circuit isconnected in series with the resistors R9 and RIB, across the exciter.The braking and accelerating rheostats are thus selectively connected tothe resistors R9 and RIO by the contactors ABI and A32. The contactmembers A383 function in part to complete an energizing circuit for theauxiliary accelerating relay 4A with the positive conductor 5. Contactmembers AB4 when closed supply positive potential to various of therelays in the accelerating and braking relay system and through thisrelay system energizes the RR'relay.

The field loss relay FL is provided with a single contact FLI whichfunctions in part to form an energizing circuit for the low voltagerelay LV.

Accelerating relay IAR is provided with the back contacts IARI and thefront contacts IAR2. The back contacts IARI provide when closed apartial energizing circuit for the relay IA. The contact members IAR2,when closed, partially complete energizing circuits for various of thaccelerating and braking relays.

Accelerating relay BAR is energized upon closing of the contacts IAR2,This relay has a back contact 3ARI which partially completes anenergizing circuit for the auxiliary relay 3A and front contacts 3AR2which when closed pick up the relay 4AR.

Accelerating relay 4AR is provided with back contacts 4ARI whichpartially connect the relay 4A to the positive conductor 5, and frontcontacts 4AR2 which when closed complete an energizing circuit to thecontacts IBR3 of the braking relay IBR.

Accelerating relay IA is provided with contact IAI which shunts theresistor RI4 in the series circuit of the speed rheostat SR.

The relay 3A is provided with contacts 3AI which shunt the resistor RI5also in series with the speed rheostat SR.

The relay 4A is provided with contacts 4AI which shunt the resistor RI2which, depending upon the position of the relay AB, inserts or removesthis resistor from a series circuit connecitII.1IlumIImlHWWWWWWWlibrarian,. M

tion with either of the braking or accelerating rheostats ER or AR.

The low voltage relay LV is provided with the contacts LVI and LV2. Thecontacts LVI complete a partial holding circuit for this low voltagerelay when they are closed, and the contacts LV2 function to completepartial energizing circults for the creep relay CR and the acceleratinand braking relay AB.

The master switch MS is provided with contacts MSI to MS5. The contactMSI is continuous through the off," creep" and run positions and isconnected to a point of positive potential. The contact MS2 extends onlythrough the off position. The contact M83 extends only through thecreep" position. The contact M84 extends only through the run position.The contact MS5 extends only through the creep" position.

For operation of the system at running speed the contacts RCI to RC4 areclosed. Thus the resistor elements R3, R5 and the speed regulatinggenerator field winding RS3 are shunted from one extremity of the bridgecircuit, and one output terminal of the bridge is shifted from the point3 to the point 3a. Similarly the resistors R4 and R8 and the speedregulating generator field winding RS2 are shunted from the otherextremity of the bridge circuit, and thus the other output terminal isshifted from the point 4 to the point 4a. The bridge circuit thusincludes the oppositely disposed field windings GSFI and RSI, and GSF2and RS4. The other two legs of the bridge circuit under these conditionscomprise the resistors RI and R2.

When the RC contactor is energized the RR relay is also energized. Thusthe field winding RCLI of the regulating generator RCL is connected inseries with a portion of the speed regulating rheostat SR and is thusenergized by the current flowing through this circuit. This estabiishesthe pattern voltage for the running conditions for the current limitingregulating generator RCL. The creep contactor CR under these conditionsis deenergized. Thus its contact members CRI and CR2 are open. The fieldwinding RS5 with its series resistor R" is thus connected in parallelwith the field winding RCL2 and its series resistor RIB. Since thecontact members CR2 are open, the field windings RS1 and RCLI and theseries resistor RI! are connected across the motor armature terminals.Thus both of these fields are energized.

For creeping operation of the control system the run contactor RC isdeenergized and the contacts RCI to RC4 thereof are open. This insertsthe resistors R3 and R5 and the field winding RS3 and the resistors R4and R5 and the field winding RS2. The bridge circuit for this slow speedof operation of the motor is thus recalibrated and the sensitivity ofthe speed regulating generator RS increased because of the increase inampere turns of its field windings. The creeping relay CR for the slowspeed is energized. Thus its contact CRI shunts the resistor RI! andrecalibrates the field RS5. Closure of the contact members CR2 shuntsthe field RCL! of the speed regulating generator and its series resistorRI! and thus in effect removes this field from the system. At the sametime the contact members CR3 and CR4, upon closing, recalibrate thecircuit including the speed controlling rheostat SR and provideexcitation forthe bridge circuit only according to the drop across a.portion of the resistor RIO connected in shunt circuit with the bridge.

Before proceeding with a discussion relating to the operation of thesystem, an understanding of the characteristics of the regulatinggenerator RS should behad. This generator is of the series type. In Fig.3 of the drawings the saturation curve of a series generator is drawn.Stable operation of a series generator is obtainable only if theresistance lines of the field circuit is less than that of a linetangent to the initial substantially straight line portion of thesaturation curve. If the resistance'is higher, as indicated by the lineA-A, the generator voltage cannot build up. If the resistance is lower,the generator.

voltage will be that determined by the intersection of the resistanceline with the saturation curve, for example, points CC'. If theresistance line is exactly tangent to this curve, the generator can havean open circuit voltage equal to any of the points of tangency, forexample, the points B and B. This latter characteristic is obtained byproper selection of the resistance value of the resistor SRI.

While this would be undesirable in a standard generator, it is'an idealcharacteristic for a regulating generator such as RS. The purpose of thecontrol field windings RS1, RS5 and RS6 thus becomes that of locatingthe proper operating point of a series generator including the armaturecurrent energized and pattern fields RSI to RS4, inclusive, and thearmature winding, keeping this operating point constant for a givencontrol condition with the regulating generator supplying the requiredamount of power to the generator field windings GSFI and GSFZ. With thespeed control rheostat SR set at the desired position the electricalbridge circuit, and consequently the windings included in the legs ofthe bridge circuit, are energized with a current indicative of thedesired speed of operation of the motor. Thus the four field windingsRSI to RS4 establish the pattern voltage for the speed regulatinggenerator as well as imparting self-energizing properties thereto.

Since it is desired to regulate the speed of the motor M, an indicationof the counter E. M. F. of the motor is desired. The magnitude of themotor counter E. M. F. is an indication of the extent of the departurein speed of the motor from the desired speed. An indication of thecounter E. M. F. is obtained by differentially comparing the motorarmature terminal voltage with the IR drop of the motor armature. Thiscomparison takes place under running and creeping conditions between thedifferentially connected field RS1, which is energized according to themotor armature terminal voltage, and the field winding RS5, which isenergized by the voltage drop across the generator commutating fieldwinding GCF which carries the load current. The differential voltageresulting from this comparison, if matched by the pattern votage forrunning conditions of the field windings RSI and RS4 of the speedregulating generator, indi-- cates that the motor speed is correct forthe setting of the speed rheostat SR. If the motor speed is low, theresulting differential voltage is low and no longer matches the patternvoltage. The speed regulating generator thus generates a voltage ofsufficient magnitude which causes a current to circulate through thefield windings in the bridge circuit in such a direction as to increasethe electrical output of the main generator G to thus speed up themotor. As the motor 10 increases in speed, eventually matches thepattern voltage at which point the electrical output oi the speedregulating generator is maintained by the self-energizing propertiesthereof, thus maintaining the corrective current. If the motor speedshould for any reason be in excess of that indicated by the speedcontrolling rheostat SR, the differential voltage overbala-nces thepattern voltage and reverses the electrical output of the speedregulating generator. This holds down the excitation of the maingenerator fields GSFI and GSFZ since now the current circulatedtherethrough by the speed regulating generator 'is in opposition to thatsupplied to the bridge circuit through the speed controlling rheostat.The electrical quantity thus supplied to the motor drops and as aconsequence the motor speed drops until the difierential and patternvoltages are matched.

The current limiting regulating generator RCL in this application is notprovided with armature current energized field windings such as thespeed regulating generator. It is to be understood, however, that suchfields may be used if desired. As shown, this machine functionssubstantially in the manner of a standard generator. Its specialcharacteristics are illustrated graphically in Fig. 4. It is providedwith three control field windings RCLI, RCLZ and RCL3 of nected inseries with the speed regulating rheostat SR by the contact members RRIwhen the run contactor is energized, and thus under running conditionsadds its ampere turns to those of the winding RCLZ to increase theexcitation and consequently the electrical output of this machine. Thefield winding RCL3 being differentially connected with respect to RCLIand RCL2 subtracts its ampere turns from these two windings.

Since it is desired to limit the current in the motor generator seriesarmature circuit within a predetermined maximum value, some means mustbe provided for controlling the electrical output of the currentlimiting regulating generator RCL such that it is effective to limit themotor armature current as the predetermined maximum value is approachedand at other times have no effect upon the systems. This is accomplished by means of the rectifier network including the resistors R9 andRIO and the rectifiers 6 and I. The voltage drop across the resistor R9for either running or creeping is opposed to the voltage generated bythe current limiting generator. Currents produced by the voltage dropacross the resistor R9 are prevented from circulating back through thearmature o! the current limiting generator by means of. the rectifiers 6and I. These rectifiers are so disposed in the circuit that only thecurrents generated bythe current limiting generator are circulatedtherethrough. Thus no current flows through this circuit duringacceleration until the generated voltage exceeds the blocking voltageproduced by the resistor R9. As a result, the speed regulating generatorexcitation of the field winding RS6 which is connected in series withthe armature of the current limiting generator and the series resistorR8 is zero until the motor armature currents approach the selectedmaximum value. The efiect of the field winding RS6 on the speedregulating generator tends to reverse the output of the speed regulatinggenerator and it is eflective under certain the diflerential voltage vconsequently the field RCL2 which is the pattern field for the RCLgenerator is energized. Thus the excitation of this machine isincreased. This immediately causes the RCL generator to produce avoltage of sufilcient magnitude to overcome the blocking voltage whichdetermines the points 01 and O: which represent the ampere turnsnecessary to overcome the blocking voltage. As a result the winding R6is energized in such a direction as to reduce the effective ampere turnsthe speed regulating generator RS and if necessary reverse itselectrical output to hold the starting currents down. As the motoraccelerates the voltage across its motor armature terminals increasesand as follows the excitation of the differential winding RCL3 increasesand reduces the total ampere turns of th machine. This continues untilthe RCL2 and RCLS fields are neutralized at which time the output of thecurrent limiting generator drops to zero. The current field winding RCL2might be used alone under running conditions to control the RCLgenerator, however, with such an arrangement the current in the motorarmature circuit might overshoot safe values by a considerable marginbefore correction took place. With the preceding arrangement correctionis initiated as these conditions are approached.

For creeping operation the winding RCL2 alone is sufficient since thereis little, it any, possibility of the occurrence of dangerously highcurrents. Sufilcient correction will take place for the short durationnecessary by the action of the field RCL2.

The operation of the system for running speeds of the motor is asfollows:

The exciter E, the current regulating limiting generator RCL, the speedregulating generator RS and the main generator G are first brought up tonormal operating speed by the prime mover (not illustrated). The excitervoltage is thus applied across the motor field winding MF. As theexciter voltage builds up the relay FL is sufficiently energized that itmay pick up and close its contact members FLI. With the master switch inthe oil position as shown, the low voltage relay LV is energized througha circuit across the positive and negative terminals indicated, whichincludes the contacts MSI and MS! of the mast-er switch and the contactmembers FLI. When the LV relay picks up, its contact members LVI and LV2close. Pressing of the reset" button, so designated in the drawings,connects the coil of the main contactor MC across the positive andnegative terminals indicated. This contactor then moves to its energizedposition and closes its contact members MC, thereby completing the motorgenerator series armature circuit. Since the bridge circuit includingthe generator fields GSFI and GSF2 is not yet connected across theexciter buses because the contacts CR4 and RRI selectively completingthis circuit are open, no current fiows in the motor armature circuit.The contact members MC2' open and thus open the discharge circuitincluding the resistor R1 for the electrical bridge circuit. The contactmembers M03 close and complete a partial energizing circuit for the coilof relay DFR. The contact members MCl close and complete a holdingcircuit for the low voltage relay LV which also includes the contacts PMand LVI. Thus the master switch MS may be removed from its ofi" positionto either the creep" or "run" positions without the low voltage relaybeing deenergized. The contact members M05 close and shunt the "resetbutton. Thus when the reset" button is released and its contacts open,the coil of the main contactor remains energized through the "stop" pushbutton which is normally closed and the now closed contacts M05.

Operation 01' the master switch to the "run" position completes anenergizing circuit through its contact members MSI and M84 and thecontact members CR5 for the coil or the run contactor RC. This contactorpicks up and closes its contact members RCI to RC4, inclusive, shuntingthe field windings RS2 and RS! and the associated resistors from theelectrical bridge circuit. The back contact members RC5 open. The frontcon tact members RC6 close and thus energize the conductor 5.

Energization of the conductor 5 energizes the coil of the difierentialfield relay DF which picks up openin its contact members DFI, thusdisconnecting the generator differential field GDP. The contact membersDFI close to complete a partial energizing circuit for the RR relay.When the contact members DF3 close, an energizing circuit for the relayAB is completed through the contact members LV2, DF3, the coil of therelay AB, the contact member CR6 and the master switch contacts M84 andMSI. At the same time the DFR relay picks up through its connection withthe now energized conductor 5 by means of the contacts M03. This closesits contact members DFRI and connects the armature oi the speedregulating generator RS across the output terminals 3a and 4a of theelectrical bridge.

When the AB relay picks up, its contact members ABI open disconnectingthe braking rheostat, and its contact members AB! close insert ing theaccelerating rheostat in series with the resistor RiZ. Meanwhile, therelay IA has been energized and its contact members Al closed to shuntthe resistor R12 through a circuit from the conductor 5, which includesthe contact members AARI, lRBl to the coil of the relay IA. The bridgecircuit is, however, not yet connected since the RR relay is notenergized. The contact members AB3 complete a holding circuit inparallel with the energizing circuit established in part by the contactmembers lBRl for the coil of the relay 4A. The contact members AB, uponclosing, connect the coil of the relay IBR across the positive andnegative terminals indicated and similarly energize through the yetclosed contacts RR3, the coil of the relay IAR. Both of these relayspick up. The relay IAR opens its contact members IARI, opening theenergizing circuit for the coil of the relay IA, and closes its contactmembers IARZ. Closing of the contacts IAR! energizes the coil of therelay lAR. This relay thus opens its contacts IIARI to open theenergizing circuit for the coil of the relay 3A and closes its contactmembers 3AR2 which energize the coil of the relay AR. Relay AR opens itscontact members IARI and closes its contacts ARJ. Opening oi the contactmembers IARI disconnects the coil of the relay 4A from the conductor 5.The contact members lAl thus open and insert As the motor acceleratesand the various accelerating relays function, the accelerating rheostatcircuit is recalibrated by shunting the resistor Rl2 in seriestherewith. The blocking potential for the current limiting generatoracross the resistor R9 is thus increased which correspondingly increasesthe maximum currents which may be attained in the motor armature circuitbefore the current limiting generator begins to function. This ispermissible since the current limitation provided for the initialstarting period is lower than that necessary to protect the system. Thusno damage to the system from excessive currents may result. Therecalibrating which then occurs effectively increases the value of themaximum' currents which may be attained to such limits that prolongedoperation of the system just below this maximum value will in no waydamage the system. Closure of the contact members 4AR2' completes anenergizing circuit through the contact members IBR3, which are nowclosed since the relay [ER is energized, to the coil of the relay 3BR.This relay now closes its contact members 3BRI and 3BR2. At the sametime a circuit is completed through the contact members iBR3 and IBRZ tothe coil of the relay. 4BR. The relay 4BR opens its contacts 4BRI, whichpreviously cooperate with the contacts 4AR| to connect the coil of relay4A to the conductor 5. Closure of the contact members 3BR! and 3BR2completes an energizingcircuit for the run relay RR which includes thecontact members AB l, IAR2, 3AR2, 4AR2, IBR3, IBR2, 3BRI, 3BR2, the coilof the relay RR and the contact members DF2. This energizes the RR relaywhich then closes its contact members RRI to connect the pattern fieldfor the current limiting generator in series with the speed controllingrheostat SR and energize thecbridge circuit. The motor accelerates withthe resistors RI! and RH in series with the accelerating rheostat AR.Thus a current flows through this field winding of a magnitude dependingupon the setting of the speed rheostat SR. Closure of the contactmembers RR2 provides a holding circuit in conjunction with the contacts3BR2 and DFZ for the coil of the relay RR. Opening of the contactmembers RR3 deenergizes the relay IAR. This relay now closes its contactmembers IARI after a short time interval and opens its contact membersIAR2. The relay IA is now picked up to close its contact members lAlthrough a circuit which includes the contact members RR2 and the contactmembers IARI and shunt the resistor RH from the speed rheostat circuit.The motor thus increases in speed. The energizing circuit for the relay3AR is open at the contact members IARZ. After a short time intervalthis relay drops out and closes its contact members SARI which, inconjunction with the yet closed contacts IBRI, energizes the relay 3Awhich closes its contacts 3A! and shunts the next resistor step Rl5 iromthe speed controlling rheostat circuit to further increase the motorspeed. The contact members 3AR2 now being open deenergize the relay 4ARwhich after a short time interval drops out independent of the masterswitch position. When and again closes its contact members lARl. Sincethe contact members ABS are closed, the coil of the relay 4A is againconnected to the conductor 5. This relay now picks up and closes itscontacts 4A! to again shunt the resistor R to increase the drop acrossthe resistor R9. Opening of the contact members lARI does not deenergizethe 3BR and 4BR relays since these relays are held in through the mediumof the contact members RR! and SBRI.

As the voltage across the generator G and the voltage across the motor Mbuild up, the relay IVR first picks up and closes its contact membersIVRI. This energizes the timing relay TR which closes its contactmembers TRI to complete an energizing circuit for the conductor 5 whichis the motor approa :hes say 30% of maximum running speed the relay ZVRpicks up, thus shunting the resistor R|3 from its series relationshipwith the accelerating rheostat AR, thus further increasing the voltageacross the resistor R9 and further increasing permissible motor armaturecurrents. When the voltage across the generator armature terminalsreaches a predetermined value the relay 3VR picks up and establishes aparallel energizing circuit for the coil of the relay TR. In a similarmanner, once the voltage across the motor commutating field has reacheda certain value the relay R picks up and establishes a third energizingcircuit for the relay TR. The motor is thus accelerated to the speedselected by the setting of the speed rheostat SR and the generatorfields GSFI and GSF2 are additionally excited during this acceleratingperiod by the speed regulating generator.

Should the currents existing in the motor armature circuit besuiiioiently high, the excitation of the current limiting field windingRCL2 added to that of the winding RCLI finally reaches a value such thatthe resulting ampere turns of the three windings produces a generatedvoltage which is greater than the opposed voltage or blocking potentialof the resistor R9. The total excitation of the speed regulatinggenerator RS is thus reduced to limit the current circulating in themotor generator armature circuit. Thus the motor is accelerated at themaximum permissible rate without the possibility of overloading themotor generator armature circuit.

Movement of the master switch to its off position immediatelydeenergizes the A B relay. This relay now closes its contact members ABIand opens its contact members AB2,thereby dis connecting theaccelerating rheostat AR and connecting the braking rheostat ER inseries with the resistor R9. Since the relay 4BR is yet energized,opening of the contact members AB3 deenergizes the relay 4A which nowinserts the resistor R12 in series with the braking rheostat and itsseries resistor RI I. As a result the voltage across the resistor R9 isreduced and the current resulting from regeneration during brakingperiods of the motor reduced. The contact members AB4 deenergize therelay IBR, which through the opening of the contact members IBRI,deenergize the relay 3A to correspondingly decrease, through theinsertion of the resistor R15 in serieswith the speed rheostat, theexcitation of the main generator fields GSFI and GSFZ. This also reducesthe pattern voltage of the speed regulating generator since theexcitation of the self -energizing fields RSI and RS4 is correspondinglydecreased. Opening of the contact members IBRZ deenergizes the relay 3BRwhich after braking rheostat to slightly increase the permis-- siblecurrent in the motor generator series armature circuit. Opening of thecontact members 3BR2 deenergizes the coil of the relay RR. This relaydrops out and opens its contact members RRI and disconnects the bridgecircuit from its supply of electrical energy. As the voltage across themotor and the generator decreases, the relays R, I VR, 2VR, and 3VR dropout in a sequence 'which depends upon the nature of the voltagedecrease. This eventually deenergizes the timing relay TR which opensits contact members TRI which now provide the only source of electricalenergy supply for the conductor 5. The relays DF' and DFR therefore dropout and respectively connectthe differential field GDF across the maingenerator to kill the residual voltage of this machine, and disconnectthe speed regulating generator RS from the terminals 30 and 4a of theelectrical bridge circuit. If at any time during the period ofdeceleration of the motor the regenerative currents in the motorarmature circuit should rise above the established value, the currentlimiting regulating generator will limit the braking current. Underregenerative conditions the field winding RCL2 is reversely energizedand the drop across the resistor RIIl is, under this condition,eflective to block the output of the current limiting generator. As theexcitation of the field RCL2 increases, the generated voltageoverbalances the blocking voltage and the field winding RS6 excites thespeed regulating generator RS in such a manner that the electricaloutput of this machine tends to maintain the voltage of the maingenerator. The regenerative voltage is thus opposed by the voltagegenerated by the main generator and the currents in the motor armaturecircuit maintained within permissible values.

Operation of the master switch to the creep position energizes the coilof the creep relay CR through a circuit including the master switchcontacts MSI and M35, the contact members RRC! now closed, the coil ofthe relay CR, the contact members DF3 and the contact members LVZ. Thecoils of the DF and DFR relays are energized from the conductor which isnow energized through the master switch contacts MSI and M33 and theback contacts RC5. When the creep relay picks up its contact members CRIshunt the resistor RII from its series connection with the winding RS5of the speed regulating generator. This recalibrates the field windingwhich is excited according to the motor armature currents and in effectincreases its sensitivity. In a similar manner the contact members CR2shunt the difierential field winding RCL3 and its series resistor RI 9from the series connection with the field winding RSI. The difierentialfield winding RSI thus has the full motor armature terminal voltageapplied thereacross. Since the run cont-actor RC is not energized duringcreeping operation of the motor,'the windings RS2 and RS3 of the speedregulating generator, and their associated resistors, are connected inthe electrical bridge circuit. This in effect increases the patternvoltage for a given excitation as well as the self-energizing 16properties of the machine. The contact members CR3, in conjunction withthe contact members CR4, connect the speed controlling rheostat and itsseries connected resistor R to RIG across the exciter. The contactmembers CR3 function additionally to connect a portion of the resistorRIB across the input terminals of the electrical bridge circuit. Thisbridge circuit is thus excited in efi'ect by the voltage drop across thementioned portion of the resistor RIG. A fine control or the excitingvoltage supplied to the bridge is thus obtained and this voltage may bevaried, depending upon the resistance of the speed controlling rheostat.In the instant case this resistance is a fixed value. The resistance,however, may be varied in any well known manner if desired. The contactmembers CR5 open the energizing circuit for v the coil of the relay RC.The contact members CR6 open the energizing circuit for the relay AB.Since the contact members ABI are open, the accelerating and brakingrelay system is not operable. Thus the motor is accelerated and operatedwith all the resistors RN to RIG connected in series with the speedcontrolling rheostat SR. The contact members ABI, being closed, connectthe braking rheostat BR in series with the resistors RI2, RII, R9 andRIO. The voltage drops across the resistors R! and RI'II in this caseare controlled during accelerating and braking by the braking rheostatalone. Since the contact members RRI of the run relay RR are open duringcreeping of the motor, the field RCLI of the current limiting generatordoes not function. Hence the entire control 01. the current limitinggenerator is effected by the field RCL2. Since the creeping speed is somuch lower than the running speed, that is, it is only about 1% orpossibly slightly higher of the running speed, the accelerating andbraking control previously described is unnecessary. The relay IVR picksup initially and energizes the timing relay TR, thus establishing asecond energizing circuit for the conductor 5. The relays R and 3VR pickup, respectively, when the motor commutator field and the generatorterminal voltage is sufiicient.

The variable voltage drive of this invention provides automatic overloadcurrent protection which results not in a time wasting shut-down of thesystem, but which limits the currents within permissible values, therebyproviding maximum acceleration of the motor for the given mechanicalload it is driving. Further, this automatic overload protection isprovided by means of a simple small rotating regulator which requires aminimum of maintenance eiTort, and which for the most part eliminatesthe complicated relay or contactor systems which function in thiscapacity.

The foregoing disclosure and the showings made in the drawings aremerely illustrative of the principles of this invention and are not tobe interpreted in a limiting sense. The only limitations are to bedetermined from the scope of the appended claims.

We claim as our invention:

1. In a variable voltage drive, the combination of, a motor, a maingenerator for energizing the motor, field windings for the maingenerator, an electrical bridge circuit, means for supplying electricalenergy to the electrical bridge circuit, certain of the field windingsof the main generator being connected in opposite legs of the electricalbridge circuit such that the winding ampere turns in the opposite bridgelegs are 17 I equal, a first regulating generator responsive toelectrical quantities of the motor for controlling the excitation ofsaid certain of the field wind ings, a second regulating generatorresponsive to motor armature currents for controlling the firstregulating generator, and means for producing a voltage opposing thevoltage generated by the second regulating generator to prevent thevoltage generated by the second regulating generator from effecting thefirst regulating generator until the generated voltage exceeds theopposed voltage.

2. In a variable voltage drive the combination of,v a motor, a maingenerator. for energizing the motor, field windings forthe maingenerator, an electrical bridge circuit, means for supplying electricalenergy to the electrical bridge circuit, a field winding of the maingenerator connected in each of two opposite legs ofthe electrical bridge1 circuit, a first regulating generator responsive to electricalquantities of the motor, connected to circulate its output currentsthrough the field windings of the main generator which are connected inthe electrical bridge circuit, a field winding for the first regulatinggenerator, a second regulating generator having an armature winding anda field winding, the armature winding of the secondregulating generatorbeing connected in series with the field winding of the first regulatinggenerator, means for energizing the field winding of the secondregulating generator in accordance with the currents in the armature ofsaid motor, and means including a resistor and rectifier network forpreventing the electrical output of the second regulating generator fromcirculating through the field winding of the first regulating generatoruntil'apredetermined motor armature current is reached.

3. In a variable voltage drive, the combination of, a motor, a maingenerator for supplying electrical energyto the motor, field windingsfor the main generator, an electrical bridge circuit, means forsupplying electrical energy to the bridge circuit, a-field winding ofthe main generator connected in each of two opposite legs of the bridgecircuit, afirst regulating generator having an armature winding and agroup of field windings, circuit means connecting said armature windingof the regulating generator across the output terminals of theelectrical bridge circuit, a pair of field windingsof the firstregulating generator being connected in series with each main generatorfield winding in opposite legs of the electrical bridge circuit, asecond regulating generator having an armature winding and a controlfield winding, circuit means including a rectifier and resistor networkconnecting the armature winding of the second regulating generatorinseries with a field winding of the first regulating generator, meansforsupplying electrical energy to the rectifier and resistor network,circuit means connecting the field winding of the second regulatinggenerator. to be energized in accordance with the currents flowingthrough the armature of said motor. i

4. In a variable voltage drive, the combination of, a motor having anarmature winding and 'a field winding, a main generator having anarmature' winding and a pair of control field windings, circuit meansconnecting the armature Winding of the main generator in series circuitrelation with the armature winding of the motor, an electrical bridgecircuit, each of said pair of field windings for the main generatorbeing connected in an opposite leg of the electricalbridge circult,means for supplying electrical energy to the electrical bridge circuitand to said motor field winding, a first regulating generator having anarmature winding and seven field windings, circuit means connecting thearmature winding of the first regulating generator across the outputterminals of the electrical bridge circuit, four of said field windingsfor the first regulating generator being connected in the electricalbridge circuit, two in series with each main generator field winding,first control means for shunting a first regulating generator fieldwinding from each of the opposite bridge circuit legs thus leaving onein serieswith each main generator field winding,

a fifth first regulating generator field winding connected across themotor armature terminals, the fifth field winding being a differentialfield, a sixth field winding connected to be energized in accordancewith thecurrents traversing the motor generator series armature circuit,a second regulating generator having an armature windcircuit meansincluding a rectifier and'resistor network connecting a seventh fieldwinding of the first regulating generator in series with the armaturewinding of the second regulating generator, said rectifier and resistornetwork being connected to said means for supplying electrical energyand being operable to preventan electrical output generated by thesecond regulating generator from circulating through said seventh fieldwinding until said electrical output reaches a predetermined minimumvalue.

5. In a variable voltage drive, the combination of, a motor, a maingenerator for energizing the motor, a first regulating generatorresponsive to electrical quantities of the motor for controlling theelectrical output of the main generator, a second regulating generatorresponsive to an electrical quantity of the motor for controlling theelectrical output of the first regulating generator, and control meansfor decreasing the sensitivity of the first regulating generator whileincreasing the sensitivity of 'the second regulating generator to efiectone speedof operation of the motor and for increasing the sensitivity ofsaid first regulating generator while decreasing the sensitivity ofsecond regulating generator for effecting another speed of operation ofthe motor.

6. In a variable voltage drive, the combination of, a motor, a maingenerator for energizing the motor, field winding meansfor the maingenerator, means for supplying electrical energy to said field windingmeans, a first regulating generator responsive to electrical quantitiesof the motor for controlling the excitation of the main generator fieldwinding means, field winding means for the first regulating generator, asecond regulating generator responsive to an electrical quantity of themotor for exciting a portion of the field winding means of the secondregulating generator only when said electrical quantity of said motor towhich said second regulating generator responds exceeds a predeterminedvalue, field winding means for the second regulating generator, andcontrol means for shunting certain portions of the field winding meansof the first regulating generator while inserting certain portions ofthe field winding means of the second regulating generator to efiect onespeed of operation of the motor and for inserting said certain portionsof the field winding means for the first regulating generator whiledisconnecting said certain portion of the field winding means for thesecond regulating generator to efiect another speed of operation of themotor.

7. In a variable voltage drive, the combination of, a motor, having anarmature winding and a field winding, a main generator having anarmature winding and a pair of control field windings, circuit meansconnecting the armature windings of the main generator and motor inseries circuit relationship, an electrical bridge circuit, each of saidpair of field windings for the main generator being connected in anopposite leg of the electrical bridge circuit, means for supplyingelectrical energy to said electrical bridge circuit and to the motorfield winding, a first regulating generator having an armature windingand seven field windings, a second regulating generator having anarmature winding and three field windings, circuit means connecting thearmature winding of the first regulating generator across the outputterminals of the electrical bridge circuit, four of said field windingsfor the first regulating generator being connected in the electricalbridge circuit, two in series with each main generatcr field winding,first control means for shunting a first regulating generator fieldwinding from each of the opposite bridge circuit legs thus leaving onein series with each main generator field winding, 3, fifth firstregulating generator field winding connected in series with a firstfield winding of the second regulating generator across the motorarmature terminals, the fifth field winding of the first regulatinggenerator being a differential field, second control means for shuntingthe first field winding of the second regulating generator from itsseries connection with the said fifth field winding of the firstregulating generator, a sixth field winding for the first regulatinggenerator connected to be energized in accordance with the currenttraversing the motor generator series armature circuit, a second fieldwinding of the second regulating generator being connected to beenergized in accordance with currents traversing the motor generatorseries armature circuit, said first field winding for the secondregulating generator being difierentially connected with respect to saidsecond field winding for the second regulating generator, third controlmeans for connecting the third field winding for the second regulatinggenerator to be energized in accordance with the electrical energysupplied to the electrical bridge circuit, circuit means including arectifier and resistor network connecting a seventh field winding of thefirst regulating generator in series with the armature winding of thesecond regulating generator, said rectifier and resistor network beingconnected to said means for supplying electrical energy and beingoperable to prevent an electrical output of the second regulatinggenerator from circulating through said seventh field winding until saidelectrical output reaches a predetermined value, and a master controllerfor selectively effecting operation of the first and third control meansto obtain one motor speed and the second control means for another motorspeed.

8. In a variable voltage drive the combination of, a motor, a generatorfor energizing the motor, first means for varying the speed of saidmotor, second means for maintaining the speed of said motor constant forany selected speed thereof, third means for limiting the motor current,fourth means for progressively controlling said third means formaintaining said motor current low during starting thereof and providingan increase thereof as said motor accelerates to the selected operatingspeed, and means for effectively minimizing the effect of said thirdmeans on said motor current over selected low speed ranges of saidmotor.

GEORGE E. KING. WILLIAM H. FORMHALS.

